1. Field of the Invention
The present invention relates in general terms to the placing of bead cores on a drum for building carcasses for tyres.
2. Description of the Related Art
A tyre generally comprises a carcass structure including at least a carcass ply, operatively associated with a pair of annular reinforcing structures suitable for matching with a mounting rim, each annular reinforcing structure comprising at least a bead wire, and folded around them, at least a belt structure at a radially external position with respect to said at least a carcass ply, and a tread band at a radially external position with respect to the belt structure.
During the building of a tyre, a cylindrical carcass ply comprising one or more reinforcing plies (containing cords embedded in a layer of an elastomeric material) is formed on a suitable drum. At least two bead cores are then positioned on this sleeve, coaxially with the axis of rotation of the drum, which coincides with the axis of rotation of the tyre, in axially separated positions.
The cylindrical carcass ply constructed on the said drum will then be shaped into a torus, possibly after removal from the abovementioned drum (e.g. by contracting the drum).
The bead wire is a hoop of metal wires having different cross sections: for example, square, rectangular or hexagonal. The so-called Alderfer type has wires laid axially side by side and radially on top of each other, and they are rubberized.
The twisted type has a basically round cross section, and the metal wires that form it are wound together around a central core. This type of bead wire is typically not rubberized.
The process of assembling bead wires onto the carcass ply, by an automatic or semi-automatic procedure, generally involves the following steps:                a) the bead wire is located on a device generally known as a loader;        b) the bead wire is gripped by a device generally known as a gripper;        c) the bead wire is positioned on the carcass ply.        
Step a) can be done either manually by an operator or automatically. Either way, the bead wire can be made to swing when positioned on the loader device. It is important in this step for the bead wire to stop all swinging motion quickly and be stationary, and if possible already coaxial with the building drum, when presented to the members of the gripper device, so that it can be gripped automatically, quickly and correctly.
The gripper device is often designed in such a way that its parts maintain/position the bead wire coaxial with the drum axis, so that it can be correctly and easily positioned on the carcass ply in step c).
An important requirement for bead wire placing systems relates to the flexibility of production changes. Bead wires come in a wide range of diameter measurements, depending on which type of tyre is being built. It is advantageous for economical and production reasons to find a bead wire placing system capable of controlling bead wires of different diameters with the minimum of replacements or adjustments of its devices.
Many tyre bead wire placing systems have been proposed. Many of these grasp the bead wire along its radially inner surface and/or employ magnetic means, in this case acting, for example, on its lateral surface. In particular, this depends on the fact that the bead wires are often loaded and positioned on the drum with their filler (a ring of elastomeric material of circular or roughly triangular section) already assembled on the radially outer surface of the bead wire.
For example, U.S. Pat. No. 5,051,149 (in the name of Bridgestone Corporation) describes, among other things, devices for loading, gripping and positioning bead wires to which the filler is already assembled.
The said positioning device consists of a pair of support rings, attached to the inner surface of which is a plurality of radially moveable gripper devices. Each of the said gripper devices possesses, attached in a position corresponding to the portion facing the building drum, a curved segment which itself comprises a plurality of magnets embedded in its inner surface so as to grip a bead wire plus its filler.
In addition, each support ring is connected to a cylinder with a piston whose end portion is connected to the abovementioned gripper devices. When the said piston extends, all the gripper devices are moved synchronously in a radially inward direction, and out again, by means of a complex lever system.
The said loader device consists of two specular axially opposed units mounted on the outer end portions of the shaft that carries the building drum. Each of the said units comprises mechanical gripper devices which, when moved radially outward, encounter the circumferentially inner surface of a bead wire supplied by an operator.
When the said loader devices come into relation with the corresponding positioning devices, the magnets present in the latter capture the bead wire and hold it in exact alignment with the axis of the building drum.
The Applicant has noticed that the bead wire placing systems proposed in the prior art are very complex, bulky and costly.
To be specific, the amount of radial space required by these known systems is linked with their ability to accept a wide range of bead wire measurements. This flexibility, which is desirable when it comes to changing production, also affects the structure and disposition of the gripper units that must be adapted or even replaced when different bead wire measurements are required, and this obviously involves slowing down the rate of production and, in the case of replacement, necessitates somewhere to store the various parts.
Furthermore, producing a self-centring radial movement usually demands, as seen above, a complex system of gears and levers that weigh down the entire system and increase its cost.